VII' Other Applications

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Introduction to Nanotechnology
VII. Other Applications M. Meyyappan Director,
Center for Nanotechnology NASA Ames Research
Center Moffett Field, CA 94035 email
mmeyyappan_at_mail.arc.nasa.gov web
http//www.ipt.arc.nasa.gov
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Outline
Chemical Sensor Biosensor Field Emission
Devices Thermoelectric Devices
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Chemical Sensor
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The Earliest Chemical Sensor in Human History
Canary in a Coal Mine
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What Do We Expect from a Well-Designed Sensor
System?
First, a single device has no value. We need a
system consisting of - Sensor or sensor
array - Preconcentrator (almost always
needed) - Micropump? Microfan? - Sample
handling, delivery, fluidics - Signal
processing unit - Readout unit (data
acquisition, processing, storage) - Interface
control I/O - Integration of the
above Criteria for Selection/Performance
- Sensitivity (ppm to ppb) - Absolute
discrimination - Small package (size,
mass) - Low power consumption - Rugged,
reliable - Preferably, a technology that is
adaptable to different platforms - Amenable
for sensor network or sensor web when needed
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Why Nanomaterials/Nanosensors?
Compared to existing systems, potential exists
to improve sensitivity limits, and certainly
size and power needs Why? Nanomaterials have
a large surface area. Example SWCNTs have a
surface area 1600 m2/gm which translates to
the size of a football field for only 4
gm. Large surface area large
adsorption rates for gases and vapors
changes some measurable properties of the
nanomaterial basis for
sensing - Dielectric - Capacitance - Condu
ctance - Deflection of a cantilever - -
4 grams
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Various Carbon Nanostructures
Single-walled carbon nanotubes
(SWCNTs) - Either a single tube or a film used
in the device - Vapor or gas adsorption on
the material leading to a measurable property
change - Multiwalled carbon nanotubes (MWCNTs)
may not be as good Vertically Oriented Carbon
Nanofibers (CNFs) - Multiwalled carbon
nanofibers - Walls are not parallel, instead
bamboo-like - Individual, free-standing,
vertical - Function as nanoelectrode, support a
probe molecule at the tip amenable for
interaction with target - Nanoelectrode array
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Carbon Nanotube Chemical Force Sensor
SWCNTs and MWCNTs, because of their mechanical
properties and the ability to reversibly buckle,
have been regularly used as probes in AFM,
SPM Lieber group (Harvard) covalently modified
the ends of such probes with chemically or
biologically active functionalities - First
introduced - COOH group by acid oxidation and
then attached an amine molecule RNH2. Such
an approach was proposed for chemical mapping.
Similar to the work by Majumdar group (UC
Berkeley) with microfabricated multiple
cantilevers for cancer detection Not much
progress in a practical sense has been reported.
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Sensing Via Monitoring Dielectric Properties of
SWCNTs
Concept Shift in the resonant frequency,
induced by gas/vapor adsorption, is highly
specific to the adsorbed species (Rao Group,
Clemson U., J. Appl. Phys., Vol. 97,
2005). Resonant frequency fo is related to
dielectric constant. Shift ?f is measured in the
operational microwave regime. To date, ?f
shift has been measured for 1500 ppm of
individual doses of He, O2, CO, NH3,
Bromopropane Challenges - Demonstration of
sensitivity - Discrimination
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SWCNT Capacitor as Chemical Sensor
The NRL group showed that the capacitance of
the SWCNTs is sensitive to a broad class of
chemical vapors. They constructed a SWCNT
chemicapacitor with chemoselective coatings and
tested various vapors (benzene, toulene,
dinitrotoulene, and about 15 others, E.S. Snow
et al., Science, Vol. 307, 2005) diluted in
air. Capacitance reversal upon removal of
source is rapid Sensitivity ranges from 0.03
ppm to 2600 ppm
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Vapor/Gas Detection via Ionization from Nanotubes
Carbon nanotubes are a known electron source
and the high energy electrons can ionize the
gas or vapor and the fragments can be detected
by the usual mass selection approach. This
is essentially miniaturized mass spectrometry
concept with a novel ionization source To
date, only noble gases have been detected (Ajayan
group at RPI, also Cambridge Group, Riley et
al., Nano Lett., Vol. 3, p. 1455, 2003). Need
ultrahigh vacuum conditions
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Conductivity Change of CNTs Upon Gas/Vapor
Adsorption
Early chemical sensors were of the CHEMFET type
with SnO2 and other oxide conducting
channels Similar CNT-FETs have been tested in
the literature, exposing to NH3, NO2, etc.
change in conductivity has been observed (Kong
et al., Science, Vol. 287, 2000) Limitations
of CNT-FET - Single SWCNT is hard to transfer
or grow in situ - Even a film of SWCNTs by
controlled deposition in the channel is
complex - 3-terminal device is complex to
fabricate - Commercial sensor market is very
cost sensitive
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Advantages of SWNTs for Chemical Sensors
Single-Walled Carbon Nanotubes For Chemical
Sensors

• High surface area to volume ratio
• Electrical properties change at room temperature
• Direct electrical signal detection and readout
• High density array
• Low power consumption

Single Wall Carbon Nanotube

• Every atom in a single-walled nanotube is on
  the surface and exposed to environment
• Charge transfer or small changes in the
  charge-environment of a nanotube can cause
  drastic changes to its conductivity

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SWCNT Chemiresistor
Conventional thin film transistor approach is
complex and expensive and cheaper alternatives
are needed. Two terminal chemiresistor is
cheaper, easier to fabricate using simple
microfabrication 1. Interdigited electrode
device fabrication 2. Disperse purified
nanotubes in DMF (dimethyl formamide) 3. Solutio
n casting of CNTs across the electrodes
Jing Li et al., Nano Lett., 3, 929 (2003)
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SWCNT Sensor Performance
Sensor tested for NO2, NH3, acetone, benzene,
nitrotoulene Test condition Flow rate 400
ml/min Temperature 23 oC Purge carrier gas
N2 Sensitivity in the ppb range Selectivity
through (1) doping, (2) coating CNTs with
polymers, (3) multiplexing with signal
processing Sensor recovery slow can be
speeded up by exposing to UV light
Detection limit for NO2 is 44 ppb.
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Sensing Mechanisms
NO2
Nitrotoluene
ONO
.
e
P-type
E0
Intratube Modulation
Intertube Modulation
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Indium Oxide Nanowires for Chemical Sensors
In2O3 thin films have been known to be
sensitive to O3, Cl2, NO2, NH3, CO, H2. but
thin film devices operate at 200-600?C. Also,
sensitivity is limited by surface-to-volume
ratio In2O3 nanowires overcome these
limitations Wire diameter 10 nm, length 5 µm
produced by VLS technique were used to fabricate
a 3-terminal FET-like structure Sensitivity (
Resistance after gas exposure/Resistance before
exposure) of 106 for NO2 and 105 for
NH3. Response time (time duration for
resistance change of one order of magnitude) is
5 seconds for 100 ppm NO2 and 10 seconds for 1
NH3 Illumination of the wires with UV
accelerate the desorption to reduce the
recovery time as much as possible.
Zhou group, U. of Southern California, APL (2003).
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In2O3 nanowires (single multiple) were used
as conducting channels in a FET-like structure
with a backgate Minimum detectable
concentration is 20 ppb, comparable to carbon
nanotube based sensors of similar
fabrication Recovery is aided by a 254 nm UV
laser.
Zhou group, USC, NanoLett (2004)
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Biosensor
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CNT Based Biosensors
Probe molecules for a given target can be
attached to CNT tips for biosensor
development Electrochemical approach
requires nanoelectrode development using
PECVD grown vertical nanotubes The signal can
be amplified with metal ion mediator
oxidation catalyzed by Guanine.
High specificity Direct, fast
response High sensitivity Single molecule
and cell signal capture and detection
Courtesy Jun Li
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Electrochemical Biosensing of DNA Hybridization
Electrode
Electrode
A
C
A
G
T
C
G
C
T
Immobilization
Hybridization
Transducing into electronic signal by
electrochemical mechanism

• Transducing mechanism
• By redox of DNA bases
• By redox of metal chelate indicators or other
  intercalators
• By indicator-free mechanisms based on conducting
  polymers (polypyrrole etc.)

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Nanotube Array as High Sensitivity DNA Sensor
Each individual array electrode is electronically
addressable dia 10 to 100 nm dnn 500 nm to
2000 nm
Top View
Side View
10 to 100 mm
Immobilized with PNA or DNA Probes
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Commonly Used Carbon Electrodes
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Nanoelectrode for Sensors
Nanoscale electrodes create a dramatic
improvement in signal detection over traditional
electrodes
Traditional Macro- or Micro- Electrode
Nanoelectrode Array
Electrode
Insulator

• CNT tips are at the scale close to molecules
• Dramatically reduced background noise

Nano- Electrode
Scale difference between macro-/micro-
electrodes and molecules is tremendous Backgroun
d noise on electrode surface is therefore
significant Significant amount of target
molecules required
Multiple electrodes results in magnified signal
and desired redundance for statistical
reliability. Can be combined with other
electrocatalytic mechanism for magnified signals.
Source Jun Li
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Carbon Nanotube Electrodes at Different Densities
CNT coverage 20 (3.0x109
CNTs/cm2) Average nearest-neighbor
distance 300 nm
CNT coverage lt 1 (1x108 CNTs/cm2) Average
nearest-neighbor distance gt 1500 nm
CV in 1mM K4Fe(CN)6 in 1M KCl at 20 mV/s
J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, Q.
Ye, J. Koehne, J. Han, M. Meyyappan, Nano.
Lett., 2003, 3 (5), 597.
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Functionalization of DNA
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Electrochemical Detectionof DNA Hybridization-
by AC Voltammetry
1-2
1st
2nd and 3rd
2-3
1st, 2nd, and 3rd scan in AC voltammetry
1st 2nd scan mainly DNA signal 2nd 3rd scan
Background
Lower CNT Density Þ Lower Detection Limit
J. Li, H.T. Ng, A. Cassell, W. Fan, H. Chen, J.
Koehne, J. Han, M. Meyyappan, NanoLetters, 2003,
Vol. 3, p. 597.
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Fabrication of Genechip

• Potential applications
• Lab-on-a-chip applications
• Early cancer detection
• Infectious disease detection
• Environmental monitoring
• Pathogen detection

30 dies on a 4 Si wafer
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Detecting Biomolecular Interactions Using
Molecular Nanomechanics

• Chen, G.Y., Thundat, T. Wachter, E. A., Warmack,
  R. A., Adsorption-induced surface stress and its
  effects on resonance frequency of
  microcantilevers, J. Appl. Phys 77, pp.
  3618-3622 (1995).
• Ratierri, R. et al., Sensing of biological
  substances based on the bending of
  microfabricated cantilevers, Sensors and
  Actuators B 61, 213-217 (1999).
• Fritz, J. et al. Translating Biomolecular
  Recognition into Nanomechanics, Science 288,
  316-318 (2000).
• Wu, G. et al. Origin of nanomechanical
  cantilever motion generated from biomolecular
  interactions, PNAS 98(4), 1560-1564 (2001).

Courtesy Prof. A. Majumdar, U.C. Berkeley
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Immobilization of Single-Stranded DNA on
Cantilever
Wu, G. et al. Origin of nanomechanical
cantilever motion generated from biomolecular
interactions, PNAS 98(4), 1560-1564 (2001).
Courtesy Prof. A. Majumdar, U.C. Berkeley
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Hybridization Experiments
Wu, G. et al. Origin of nanomechanical
cantilever motion generated from biomolecular
interactions, PNAS 98(4), 1560-1564 (2001).
Courtesy Prof. A. Majumdar, U.C. Berkeley
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Prostate Specific Antigen (PSA) Detection
PSA
HSA Human Serum Albumin HP Human
Plasminogen fPSA free PSA cPSA complex PSA
Wu, G. et al., Bioassay of Prostate Specific
Antigen (PSA) Using Microcantilevers, Nature
Biotechnology (Sept., 2001)
Courtesy Prof. A. Majumdar, U.C. Berkeley
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Field Emission Devices
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Field Emission
When subjected to high E field, electrons near
the Fermi level can overcome the energy barrier
to escape to the vacuum level Compare to
thermionic emission, where the source is heated
to 1000º C to give the electrons the energy
required to overcome the surface potential
barrier Common tips Mo, Si,
diamond Applications - Cathode ray lighting
elements - Flat panel displays - Gas
discharge tubes in telecom networks - Electron
guns in electron microscopy - Microwave
amplifiers
Courtesy P. Sarrazin
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Field Emission Model
Fowler - Nordheim equation ? is work
function, ? is field enhancement factor Plot
of ln (I/V2) vs. (1/V) should be linear At low
emission levels, linearity seen in the high
field region, current saturates Critical
low threshold E field, high current density,
high emission site density (for high resolution
displays)
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Field Emission (cont.)
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Field Emission Test Apparatus
Cathode and anode enclosed in an evacuated cell
at a vacuum of 10-9 - 10-8 Torr Cathode
glass or polytetrafluoroethylene substrate with
metal- patterned lines - nanotube film
transferred to substrate or grown directly on
it Anode located 20-500 µm from
cathode Turn-on field electric-field required
to generate 1 nA - should be
small Threshold field electric field required
to yield 10 mA/cm2
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Structure of CNT Field Emitters
Nature of nanotubes (SWNTs, MWNTs,
CNFs) Clean emitting sites vs. adsorbates
(wafer vapor, oxygen) Microstructure Screen
ing effect Diode vs. triode
Current is controlled by gate voltage,
independent of acceleration voltage
High voltage required and/or gap needs to be
adjusted
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Flat Panel Displays Using Nanotubes
Working full color flat panel displays and
CRT-lighting elements using carbon nanotubes
have been demonstrated in Japan and
Korea Display - Working anode, a glass
substrate with phosphor coated ITO
stripes - Anode and cathode perpendicular to
each other to form pixels at the intersection
- Phosphors such as Y2O2S Eu (red), Zns Cu, Al
(green), ZnS Ag, Cl (blue) - 40 display
showing a uniform and stable image Lighting
Element - Phosphor screen printed on the inner
surface of the glass and backed by a thin Al
film (100 nm) to give electrical
conductivity - Lifetime testing of the lighting
element shows a lifespan over 10000 hrs.
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Thermoelectric Devices
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Thermoelectric Effects
For an open circuit electromotive force V,
Seebeck coeff. or thermoelectric power S
If the junctions are at the same temp., a
battery inserted between C and D, then a current
I would flow around the circuit. If a rate of
heating q results at one Junction A, then there
must be cooling at a rate q at the Junction
B. ?, Peltier Coefficient
q/I Thomson Coeff.
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Why Thermoelectric Refrigeration?
Cooling plate

• Reliable
• Compact
• Light weight
• Noise free

semiconductor
p
n
heat
heat
Heat sink
I
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Thermoelectric Refrigeration
ZT TS2 ?/K ? High Seebeck
coefficient ? High electrical conductivity ? to
minimize Joule heating effect (like a
metal) ? Low thermal conductivity, K, to reduce
the heat transfer between the source and the
sink (like a glass) Generally, semiconductor
materials have a much higher S than
metals - Bismuth Telluride (ZT 1.0) To
be economically competitive with compressor based
refrigerators, ZT should be gt 3
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How to Further Increase ZT?

• Low dimensional systems
• nanowires
• Conduction electron density of state ?
• Seebeck coefficient ?
• Structural constraints
• thermal conductivity ?

n-doped
p-doped
PRL 47, 16631 (1993)
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Nanowire Based Thermoelectric Element
Source Q. Ye
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Nanowires for Thermoelectric Coolers
Si nanowire
ZnO nanowires
thermal conductivity measurement
Arun Majumdar UC Berkeley